Technical Field
The present disclosure relates to a microelectromechanical device with protection for bonding and to a process for manufacturing a microelectromechanical device.
Description of the Related Art
As is known, in recent years there have been developed microelectromechanical systems (also referred to as MEMS devices) for a wide range of applications, both in the field of sensors (accelerometers, gyroscopes, pressure sensors) and in the field of mechanical and fluidic actuation (micromotors, micropumps). Devices of this type are widely used, for example, in portable electronic apparatuses, such as portable computers, laptops or ultrabooks, PDAs, tablets, cellphones, smartphones, digital audio players, photographic cameras or video cameras, and consoles for videogames, enabling considerable advantages to be achieved as regards the occupation of space, in terms of area and thickness, and consumption levels.
In most cases, a MEMS device comprises a microstructure with movable and/or deformable parts, and a control device. The control device, according to the cases, may read the mechanical configuration of the microstructure, through measurement of electrical quantities associated thereto (for example, capacitance of capacitors with movable or deformable electrodes), or else apply electrical signals for causing controlled movements or deformations of the microstructure (for example, electrostatic forces between the movable electrodes of a capacitor).
Given that the respective machining processes differ remarkably, the microstructure and the control device are in general made in distinct semiconductor dice and then encapsulated in a common package, with the required electrical connections.
The assembly of the microstructure die and of the control device die requires using adhesive materials and may comprise steps at a wafer level or at a die level. In the former case, a wafer containing the microstructure is bonded to a supporting substrate and divided into dice only subsequently, so that each microstructure is already coupled to a respective substrate portion. In the latter case, the wafer is divided into dice, which are individually picked up and placed on a respective support (“pick and place” operation).
The assembly may present critical aspects, especially in the case of the microstructure.
The adhesive material used is frequently in the form of a film that is laminated directly on the wafer containing the microstructure prior to dicing. This solution is usually preferred to laying glues, because it produces one of the best process yields. On the one hand, in fact, the film adhesives do not require steps of hot curing, and hence the production time is shorter. On the other hand, the glues laid, until curing is completed, do not guarantee that proper positioning is maintained, thus producing a higher percentage of rejects.
Even when film adhesives are used, however, some problems may arise, in particular when the microstructure dice have wide cavities or deep trenches on the side to be bonded to the support, as in the case of microphones and pressure sensors. During lamination, which requires application of pressure, or during the pick-and-place operations (when a vacuum is, instead, created), the adhesive may penetrate into the cavity until it comes into contact with the microstructure, limiting the freedom of movement thereof. Even in less critical cases, where there is no direct contact with the movable parts of the microstructure, the presence of adhesive in the cavity may alter operation of the device. In microphones, for example, the cavity serves as resonance chamber, and its characteristics define the response of the device. Clearly, if the free space in the cavity is in part occupied by foreign material, such as the adhesive, the properties of the device are modified in an unpredictable way as compared to the design conditions.